Process for preparing surface-modified biaxially oriented film of high molecular weight polyethylene

ABSTRACT

The biaxially oriented films of high molecular weight polyethylene is formed and its surface modified to provide a hydrophilic property to the resulting film. The film is prepared by melt-kneading the high molecular weight polyethylene with an ordinarily solid hydrocarbon plasticizer and a raw fabric sheet is formed from the resulting molten mixture. After the hydrocarbon plasticizer is extracted the resulting unstretched sheet is stretched under conditions providing a stretched film having a specific surface area of 70 m 2  /g or more and a fibril structure. This stretched film is then heated under standard length constraint conditions to reduce the specific surface area of the stretched film by at least 20 m 2  /g to obtain a biaxially oriented film of high molecular weight polyethylene. The surface of the film is thereafter subjected to surface modification by polymerizing a vinyl monomer having a hydrophilic group on the surface of the film whereby the film surface becomes hydrophilic, as measured by a contact angle of the film surface to a water drop at a level of not more than 90°. The biaxially oriented films are further characterized by their excellent gas permeability, surface smoothness, tensile modulus and tensile strength.

This is a division of application Ser. No. 08/108,617 filed Aug. 26,1993 now abandoned.

FIELD OF THE INVENTION

This invention relates to biaxially oriented films of high molecularweight polyethylene, said films having a non-woven gas permeablestructure composed of microfibrils, being low in static coefficient offriction as well as in kinetic coefficient of friction, excellent intensional modulus (of elasticity) as well as in tensile strength, andalso excellent in piercing strength as well as in resistance to plyseparation, and to a process for preparing the same. Further, theinvention relates also to surface-modified biaxially oriented films ofthe aforesaid high molecular weight polyethylene, said films having beensurface-treated so that the contact angle of the film surface to thewater drop becomes not more than 90°.

BACKGROUND OF THE INVENTION

Because of excellence in impact resistance, abrasion resistance andchemical resistance in comparison with general purpose polyethylene,high molecular weight polyethylene having an intrinsic viscosity [η] ofat least 5.0 dl/g has heretofore been used as engineering plastics invarious fields. It has also been investigated to form films or sheetsfrom the high molecular weight polyethylene having such excellentproperties as mentioned above.

However, because the high molecular weight polyethylene is markedly highin melt viscosity in comparison with the general-purpose polyethylene,it is difficult to form said high molecular weight polyethylene byextrusion into film or sheet in such a manner as in the case of thegeneral-purpose polyethylene. On that account, when a film or sheet isformed from the high molecular weight polyethylene, said high molecularweight polyethylene is first extruded into a rod, and the rod is thenskived to obtain a film-like product. This process is still employed andpracticed by some people, however, it is broadly true at present thatalmost all the films or sheets of the high molecular weight polyethyleneare formed by the compression molding technique. In this connection, afilm-like product obtained by skiving a rod extruded from the highmolecular weight polyethylene has its strength of about 0.05 GPa, andthis film is dense and non-permeable.

Although the films or sheets formed from the high molecular weightpolyethylene by the above-mentioned process such as the compressionmolding technique are relatively high in impact resistance as well as inabrasion resistance, it is difficult to raise the production rate, andno molded products high in tensile strength are obtainable.

Accordingly, it is a common practice for those skilled in the art toprepare films, sheets or fibers excellent in strength from the highmolecular weight polyethylene by molding a mixture comprising the highmolecular weight polyethylene and a solvent or plasticizer capable offorming a homogeneous mixture together with said polyethylene into afilm or the like.

For example, Japanese Patent L-O-P Publn. No. 177035/1982 proposes aprocess in which high molecular weight polyethylene having a molecularweight of more than one million is incorporated with a low molecularweight hydrocarbon compound to prepare a composition, and from thiscomposition a sheet is formed. In this process, however, the sheetobtained has a tensile strength of about 0.035 GPa.

In this connection, 1 GPa may be converted into about 10,000 kg/cm².

Furthermore, there are various known processes, for example, a processfor obtaining biaxially oriented films of high molecular weightpolyethylene by biaxial orientation of a mixture comprising a highmolecular weight polyethylene and a solvent or plasticizer capable offorming a homogeneous mixture with said polyethylene, a process forobtaining microporous films by extraction and removal of the solvent orplasticizer contained in the above-mentioned biaxially oriented films ofhigh molecular weight polyethylene, and a process for obtainingbiaxially oriented films of high molecular weight polyethylene, whereina sheet is formed from a mixture comprising a high molecular weightpolyethylene and a solvent or plasticizer capable of forming ahomogeneous mixture with said polyethylene, the solvent or plasticizeris extracted and removed from said sheet, and this sheet is thenbiaxially stretched.

For instance, Japanese Patent L-O-P Publn. No. 227420/1984 discloses aprocess for preparing biaxially oriented films of high molecular weightpolyethylene, characterized in that a mixture comprising a highmolecular weight polyethylene having an intrinsic viscosity [η] of notless than 5 dl/g and a hydrocarbon plasticizer having a boiling pointexceeding the melting point of said polyethylene is biaxially stretchedmore than 3 times in the lengthwise direction and more than 3 times inthe crosswise direction at a temperature in the range of from not lessthan 60° C. to less than the melting point of said polyethylene. Thispublication also teaches that axially oriented microporous films of highmolecular weight polyethylene are obtained when the hydrocarbonplasticizer is extracted and removed by means of a suitable solvent fromthe above-mentioned biaxially oriented films of high molecular weightpolyethylene.

Japanese Patent L-O-P Publn. No. 84224/1986 discloses a process forobtaining high molecular weight polyethylene films, wherein a gel-likeproduct is prepared by incorporating polyethylene having a weightaverage molecular weight of at least 400,000 with a relatively volatilesolvent which is liquid at room temperature, and this gel-like productis formed into a sheet, followed by biaxial orientation carried out at atemperature of 75° C. or higher in the lengthwise and crosswisedirections so that the draw ratio in each direction becomes 3 or more.The high molecular weight polyethylene films obtained by this process,though being smooth and lustrous, have the maximum opacity of 15%, themaximum water vapor transmission of 0.6, and are not porous.

Japanese Patent L-O-P Publn. No. 39602/1988 discloses a process for thepreparation of high molecular weight polyethylene films, characterizedin that a gel-like product is prepared by mixing polyethylene having amolecular weight of 500,000 or higher with a nonvolatile solvent such asparaffin oil, the gel-like product is formed into a sheet afteradjustment of the amount of the solvent present in said gel-like productto 80-95%, and the sheet is stretched at a temperature below 120° C. twotimes or more in monoaxial direction and biaxially stretched 10 times ormore at an areal magnification, followed by removal of the solvent.

Japanese Patent L-O-P Publn. No. 255415/1985 describes high molecularweight polyethylene films composed of polyethylene having aviscosity-average molecular weight of 400,000 or higher and having atensile strength of 0.25 GPa in every direction. This high molecularweight polyethylene film is prepared as a non-porous high-strength filmby first forming a film from a mixture comprising a high molecularweight polyethylene and an aliphatic or alicyclic compound excellent incompatibility with said polyethylene, capable of being uniformly mixedtherewith in a molten state, and capable of swelling or wetting thepolyethylene, extracting said aliphatic or alicyclic compound from saidfilm, and stretching the thus treated film at a temperature of 130°-150°C. and a stretch rate of 50%/sec. In this high molecular weightpolyethylene film, it is designed that a decrease in mechanical strengthas a non-porous high strength-film is prevented by stretching saidpolyethylene film at a specific stretch rate under application of heatas defined above, because when the aliphatic or alicyclic compound isextracted from the high molecular weight polyethylene film, theresulting film becomes a porous film, decreasing in strength.

The high molecular weight polyethylene films obtained by the processesas mentioned above., however, involved such problems that they are aptto undergo laminar separation, crease easily and poor in retention ofshape thereof. Further, there were such problems that because of beingpoor in surface smoothness, the conventional biaxially oriented films ofhigh molecular weight polyethylene sometimes cause blocking betweenthem, stick to or twine round the machine parts at the contact portionsthereof in the course of film forming operation.

In consideration of such prior art as mentioned above, the presentinventors conducted extensive researches and, as the result, they haveeventually found that biaxially oriented films of high molecular weightpolyethylene excellent in surface smoothness and tensile strength andhaving gas permeability are obtained when a raw fabric sheet is formedfrom a blend of a high molecular weight polyethylene and a solvent orplasticizer, the solvent or plasticizer is removed from the raw sheet,and further the thus treated raw sheet is biaxially stretched to obtaina film, followed by heating under specific conditions. The presentinvention has been accomplished on the basis of the above findings.

Furthermore, an excellent affinity between the film surface of saidbiaxially oriented film and water is required according to the purposefor which this film is used. Surface of the biaxially oriented films ofhigh molecular weight polyethylene may be hydrophylized in this case.

OBJECT OF THE INVENTION

The present invention has been made in light of such circumstances asmentioned above, and an object of the invention is to provide biaxiallyoriented films of high molecular weight polyethylene low in staticcoefficient of friction as well as in kinetic coefficient of friction,excellent in tensile strength, and having gas permeability, and aprocess for preparing the same, and surface-modified biaxially orientedfilms of high molecular weight polyethylene prepared by surfacetreatment of the above-mentioned biaxially oriented films of highmolecular weight polyethylene so that the contact angle of the filmsurface to the water drop becomes not more than 90°, and a process forpreparing the same.

SUMMARY OF THE INVENTION

The biaxially oriented films of high molecular weight polyethylene ofthe invention are characterized by having a nonwoven fabric-like gaspermeable structure comprising microfibrils, being formed from a highmolecular weight polyethylene having an intrinsic viscosity [η] of atleast 5.0 dl/g, and having

(i) an average pore diameter of 0.1-10 μm,

(ii) a void content of 20-70%,

(iii) a tensile modulus of not less than 0.6 GPa,

(iv) a tensile strength of not less than 0.1 GPa,

(v) a static coefficient of friction of not more than 1.0, and

(vi) a kinetic coefficient of friction of not more than 1.0.

In the biaxially oriented films of high molecular weight polyethylenehaving such characteristics as mentioned above, it is desirable thatwhen an adhesive tape is adhered to the film having the same width asthe adhesive tape and peeled off from the film in the 180° peelingmethod to make the interlaminar separation of the films occur, theinterlaminar peeling strength is not less than 45 g/cm, provided thatsaid adhesive tape has a peel strength of 300 g/cm when two of theadhesive tapes are adhered to each other at adherent surfaces and peeledoff from each other according to the 180° peeling method. Furthermore,it is desirable that the biaxially oriented films have the number ofGurley seconds of 0.1-60 seconds, said number of Gurley seconds beingdefined by time (number of second) necessary for 10 ml of air topermeate through a film surface of 1 inch in diameter under theconditions of pressure of 600 mm H₂ O and a temperature of 23° C., andhave a pierce strength of not less than 20 g.

The process for preparing the biaxially oriented films of high molecularweight polyethylene of the present invention is characterized by;

(1) melt kneading together 20-75 parts by weight of a high molecularweight polyethylene [A] having an intrinsic viscosity [η] of at least5.0 dl/g and 80-25 parts by weight of a hydrocarbon plasticizer [B]which is a solid at room temperature and capable of forming ahomogeneous mixture, and forming a raw fabric sheet from the resultingmolten mixture,

(2) extracting said hydrocarbon plasticizer at a temperature of below90° C. with a solvent [C] capable of dissolving said hydrocarbonplasticizer and removing the extracted hydrocarbon plasticizer [B] fromsaid raw fabric sheet, thereby transforming said raw fabric sheet intoan unstretched sheet in which substantially no hydrocarbon plasticizer[B] remains,

(3) stretching said unstretched sheet at a temperature of less than 135°C. in the lengthwise direction at a stretch ratio of not less than 3times and in the crosswise direction at a stretch ratio of not less than3 times, thereby preparing a stretched film having a specific surfacearea of not less than 70 m² /g and a fibril structure, and

(4) heating said stretched film at a temperature of 132°-145° C. for aperiod of 1 second to 10 minutes under a standard length constraint,thereby reducing the specific surface area of the unstretched film by 20m² /g or more.

In the process for preparing biaxially oriented films of high molecularweight polyethylene according to the invention, it is preferable thatthe hydrocarbon plasticizer [B] to be used is a paraffin wax, and thatthe stretching temperature to be employed is 90-130° C. and the stretchratio employed is not less than 4 times in the lengthwise direction andnot less than 4 times in a crosswise direction.

The surface-modified biaxially oriented films of high molecular weightpolyethylene according to the invention are composed of a high molecularweight polyethylene having an intrinsic viscosity [] of at least 5.0dl/g and have

(i) an average pore diameter of 0.1-10 μm,

(ii) a void content of 20-70%,

(iii) a tensile modulus of not less than 0.6 GPa,

(iv) a tensile strength of not less than 0.1 GPa,

(v) a static coefficient of friction of not more than 1.0,

(vi) a kinetic coefficient of friction of not more than 1.0, and

(vii) a contact angle of the film surface to the water drop of not morethan 90°, and

a nonwoven fabric-like permeable structure.

The surface modified biaxially oriented films of high molecular weightpolyethylene as illustrated above may be prepared by subjecting thesurface of the biaxially oriented films of high molecular weightpolyethylene obtained as above to surface treatment by means of oneprocedure selected from the group consisting of corona dischargetreatment, plasma discharge treatment and electron radiation treatment,and thereby making the contact angle of the film surface to the waterdrop come to be at a level of not more than 90°.

Further, the surface modified biaxially oriented films of high molecularweight polyethylene as mentioned above may also be prepared bypolymerizing a vinyl monomer having a hydrophilic group on the surfaceof the biaxially oriented films of high molecular weight polyethyleneobtained as above, and thereby making the contact angle of the filmsurface to the water drop come to be at a level of not more than 90°.

The biaxially oriented films of high molecular weight polyethylene ofthe present invention have gas permeability and also are excellent intensile strength and surface smoothness.

Further, the surface-modified biaxially oriented films of high molecularweight polyethylene of the invention have gas permeability, excellent intensile strength and surface smoothness end excellent also in affinityfor water.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a scanning type electron photomicrograph of the biaxiallyoriented film of high molecular weight polyethylene according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Illustrated below in detail are the biaxially oriented films of highmolecular weight polyethylene of the present invention and processes forpreparing the same, and the surface modified biaxially oriented films ofhigh molecular weight polyethylene films of the invention and processesfor preparing the same.

Biaxially Oriented Film Of High Molecular Weight Polyethylene

First of all, the biaxially oriented films of high molecular weightpolyethylene of the invention are illustrated.

The biaxially oriented films of high molecular weight polyethylene ofthe invention are composed of a high molecular weight polyethylenehaving an intrinsic viscosity [η] of not less than 5.0 dl/g, preferably10-30 dl/g, and have a nonwoven fabric-like structure made ofmicrofibrils which are the smallest constituent unit of the crystal ofpolyethylene.

In the sense of polyethylene filament, it is thought that themicrofibril as referred to herein is basically 5 identical with what hasbeen reported by Peterlin [Colloid and Polymer Science, Vol. 253, 809(1975)], and said microfibril is a crystalline fiber of about 10-20 nmin width. The structure of microfibril or film may be confirmed byobserving a gold-metallized test specimen by means of a scanning typeelectron microscope at magnifications of about 10,000-30,000.

FIG. 1 shows a scanning type electron photomicrograph of the biaxiallyoriented film of the invention, and as can be seen therefrom, thismicrofibril has a so-called leaf vein-like structure.

The biaxially oriented films of high molecular weight polyethylene ofthe invention are translucent films or films of white luster, and have alight transmittance of not more than 10%, preferably not more than 10%,though it depends on the thickness of film.

The light transmittance may be measured by means of a haze meter (forexample, such as instrument as ND-H67A manufactured by Nippon DenshokuK.K.) in accordance with ASTM D-1003-69 or JIS K-6714.

The biaxially oriented films of high molecular weight polyethylene ofthe invention as illustrated above desirably have an average porediameter of 0.1-10 μm, preferably 0.1-1.0 μm.

This average pore diameter may be obtained by observation of the filmunder a scanning type electron microscope.

The porosity of the film is 20-70%, preferably 30-60%.

The porosity was obtained on the basis of the following equation whereinthe thickness of a biaxially oriented film of high molecular weightpolyethylene is taken as T1, and the film thickness of said biaxiallyoriented film, when being assumed to be a dense film (the porosity is0%), is taken as T2 (a value obtained on the basis of the density of thehigh molecular polyethylene taken as 0.96 g/cm³).

    Porosity (%)=(T1-T2)/T1×100

The biaxially oriented films of high molecular weight polyethylene ofthe invention have pores, some of them penetrate through the film, andothers do not penetrate therethrough.

The films of the invention desirably have a modulus in tensile of 0.6GPa or more, preferably 1.2 GPa or more in every direction.

The modulus in tensile (tensile modulus) as referred to in the inventionis a tangent modulus.

The films of the invention desirably have a tensile strength of 0.1 GPaor more, preferably 0.15 GPa or more and especially 0.3 GPa or more inevery direction.

Further, the films of the invention desirably have a breaking extensionof 15% or more, preferably 30% or more and especially 50% or more inevery direction.

The modulus in tension and tensile strength were determined at roomtemperature (23° C.) using a tensile tester manufactured and sold byOrientec K.K. under a trade name of Tensilon (Model RTM 100). The testspecimen used in the above test is a dumbell-shaped specimen of JIS No.1, the distance between the clamps is 115 mm, and the crosshead speed is20 mm/min. The sectional area of the specimen necessary for thecalculation was obtained from a thickness and width of the specimen.

The film thickness was measured by means of a film gage tester ofMiniaxe type DH-150 manufactured and sold by Tokyo Seimitsu K.K.

The films of the invention desirably have a coefficient of staticfriction of not more than 1.0, preferably not more than 0.7, and acoefficient of kinetic friction of not more than 1.0, preferably notmore than 0.7.

The measurement of coefficient of friction was conducted in accordancewith the method specified in ASTM D1894-63. That is, the coefficient ofkinetic friction and coefficient of static friction were calculated onthe basis of resistance as determined between two films brought intocontact with each other and rubbed with each other under a definiteload.

Because of their small coefficient of static friction and smallcoefficient of kinetic friction, the biaxially oriented films of theinvention are excellent in surface smoothness, and it is rare to produceblocking between the films. Further, it is difficult to bring about sucha problem that the blocking is produced at the time when the film asformed is wound up on a wind-up roll, and it is possible to lessen theoccurrence of such a problem that at the time of continuouslytransporting the film as formed by means of a roll, said film sticks toor twines around the machine parts of said roll at a contact portion.

The biaxially oriented films of high molecular weight polyethylene ofthe invention desirably have an interlaminar peeling strength of notless than 45 g/cm, preferably not less than 50 g/cm, as measured whensaid biaxially oriented film is applied to and peeled off from anadhesive tape according to the 180° peeling method to make theinterlaminar separation occur, said adhesive tape having a peelingstrength of 300 g/cm as measured by adhering two adhesive tapes to eachother at adherent surfaces thereof and then peeled off from each otheraccording to the 180° peeling method, and said biaxially oriented filmhaving the same width as that of said adhesive tape.

The peel strength of the adhesive tape was taken as an average value ofthe peel strength obtained, according to the 180° peeling method, byapplying adhesive surfaces of two adhesive tapes to each other and thenpeeled off from each other according to the 180° peeling method, saidaverage value being an average value of peel strength of said tapemeasured during from a point at which said tape has been peeled off fromeach other for a distance of 10 mm to a point at which said tape hasbeen peeled, further, for a distance of 30 mm.

The interlaminar peeling strength of the biaxially oriented film of highmolecular weight polyethylene was taken as a force measured by applyingan adhesive tape and said biaxially oriented film having the same widthas that of said tape to each other and then peeling, according to the180° peeling method, said tape and said film off from each other, tooccur the interlaminar separation of said film.

The interlaminar peeling strength was measured at room temperature (23°C.) and a crosshead speed of 30 mm/min using a cellophane adhesive tapehaving a width of 24 mm and an adhesive force of 300 g/cm (Cellotape®LP24, a product of Nichiban K.K.) and a Tensilon tension tester(manufactured and sold by Orientec K.K.)

Such biaxially oriented films of high molecular weight polyethylene asillustrated above are excellent in resistance to interlaminar releaseproperties.

The biaxially oriented films of high molecular weight polyethyleneaccording to the invention desirably have a number of Gurley seconds of0.1-60 seconds, preferably 0.5-30 seconds and especially 1-15 seconds.

The gas permeability as referred to in the invention may be evaluated,according to JIS P8111, JIS P8117, TAPPI T479Sm-48 add ASTM D726-58, bymeans of a Gurley densitometer (for example, B type Gurley densitometerNo. 158 manufactured and sold by Toyoseiki Seisakusho K.K.). In thepresent specification, the time (the number of seconds) required for theair of 10 ml to pass through the film surface of 1 inch in diameter at apressure of 600 mmH₂ O and a temperature of 23° C., as measuredaccording to the above-mentioned method, is defined as a number ofGurley seconds.

In the invention, the films having a number of Gurley seconds of notless than 180 seconds are termed the blocked films, that is, dense andimpermeable films.

The biaxially oriented films of high molecular weight polyethylene ofthe invention desirably have a piercing strength of not less than 20 g,preferably not less than 30 g.

This piercing strength was measured at room-temperature (23° C.) and acrosshead speed of 50 mm/min using a Tensilon tensile tester(manufactured and sold by Orientec K.K.). The needle used in this testwas a sleeve blind stitch needle having a diameter of the needle pint of30 μm.

The specific surface area of the biaxially oriented films of highmolecular weight polyethylene of the invention is desirably 30-100 m²/g, preferably 50-80 m² /g.

If this specific surface area is less than 30 m² /g, a favorable gaspermeability is sometimes unobtainable, and if said specific surfacearea exceeds 100 m² /g, the surface smoothness and resistance tointerlaminar peeling of the film are often found poor.

The measurement of the specific surface area of the film may be carriedout by means of a mercury injection type porosimeter (e.g. Autoscan-33Porosimeter, a trade name of a product of Yuasa Ionics K.K.).

The degree of orientation determined by X-ray diffraction of thebiaxially oriented films of high molecular weight polyethylene of theinvention is desirably not less than 0.75, preferably not less than0.80. The crystallinity index of said films is not less than 60%,preferably not less than 65%.

The degree of orientation of the biaxially oriented films of highmolecular weight polyethylene may be determined by a method utilizing ahalf value width H° of a peak of strength distribution curve obtained byX-ray diffraction, and the value of said degree of orientation may bedefined by the following equation.

    Degree of orientation F=(90°-H°/2 )/90°

This method is described in detail by Y. Go and K. Kubo in Journal ofIndustrial Chemical, Vol. 39, 929 (1939), and is a widely used method inpractice, wherein a half value width of the strength distribution curvealong Debye ring of the strongest paratroop surface on the equatorialplane is utilized.

The measurement of the crystallinity index may be carried out by knownmethods such as a method wherein the density of the film is measured bythe density gradient method to obtain a theoretical crystal density anda theoretical amorphous density, and the crystallinity index is obtainedtherefrom by conversion, and a method wherein an amorphous halo obtainedby X-ray diffraction of the film is utilized.

The film thickness of the biaxially stretched films of the inventionhaving such physical properties as defined above, though it is electedsuitably according to the purpose for which they are used, is usuallynot more than 500 μm, preferably 100-10 m and especially 50-10 μm.

The term "film" or "films" as used in the invention is intended toinclude tapes having its length extremely longer than its width(so-called films in continuous lengths).

Process For Preparing Biaxially Oriented Films Of High Molecular WeightPolyethylene

The process for preparing the biaxially stretched films of highmolecular weight polyethylene according to the invention is illustratedhereinafter.

In the process for preparing biaxially oriented films of high molecularweight polyethylene according to the invention,

(1) 20-75 parts by weight of a high molecular weight polyethylene [A]having an intrinsic viscosity [η] of not less than 5.0 dl/g and 80-25parts by weight of a hydrocarbon plasticizer [B] capable of producing ahomogeneous mixture with said high molecular weight polyethylene aremelted and kneaded together to obtain a molten mixture, and a raw fabricsheet is formed therefrom,

(2) said hydrocarbon plasticizer [B] is removed by extraction from saidraw fabric sheet at a temperature of not more than 90° C. using asolvent capable of dissolving said hydrocarbon plasticizer [B] to obtainan unstretched sheet containing practically no residual hydrocarbonplasticizer,

(3) said unstretched sheet is stretched at a temperature of less than135° C. so that the draw ratio both in the lengthwise and crosswisedirections becomes not less than 3 times to obtain a stretched filmhaving a specific surface area of not less than 70 m² /g and a fibrilstructure, and

(4) said stretched film is heated under standard length constraint at atemperature of 132°-145° C. for 1 second to 10 minutes to reduce thespecific surface area of said stretched film by not less than 20 m² /g,whereby the desired biaxially oriented film of high molecular weight isprepared.

Preparation Of Raw Fabric Sheet

In the invention, the high molecular weight polyethylene [A] having anintrinsic viscosity [η] of at least 5.0 d/g and the hydrocarbonplasticizer [B] are melt kneaded to obtain a molten mixture, and fromthe molten mixture a raw fabric sheet is prepared.

The high molecular weight polyethylene [A] used in the invention has anintrinsic viscosity [η], as measured at 135° C. in decalin, of not lessthan 5.0 dl/g, preferably 10-30 dl/g. If the said polyethylene [A] hasan intrinsic viscosity [η] of less than 5.0 dl/g, there are some caseswhere a film having a high tensile strength is not obtained, and if, onthe other hand, said intrinsic viscosity [η] exceeds 30 dl/g, there aresome cases where it becomes difficult to prepare a homogeneous mixturewith the hydrocarbon plasticizer [B] and, moreover, the melt viscosityof the resulting mixture increases and hence the resulting mixture asintended for becomes sometimes poor in moldability.

The high molecular weight polyethylene as illustrated above may beobtained by (co)polymerization in the presence of a Ziegler catalyst ofethylene or ethylene and α-olefin of 3-9 carbons. The α-olefin of 3-9carbon atoms used herein includes propylene, 1-butene, 1-pentene,4-methyl-1-pentene, 1-hexene and 1-butene. Among these α-olefins,preferred are propylene and 1-butene. The high molecular weightpolyethylene desirably contains the structural unit derived from theα-olefin of 3-9 carbon atoms in an amount of not more than 5% by weight.

The high molecular weight polyethylene [A] used in the invention maycontain various additives commonly used in preparing polyolefins, suchas heat stabilizers, weathering stabilizers, lubricants, anti-blockingagents, slip agents, pigments, dyes, inorganic fillers and the like tosuch an extent that nothing mars the accomplishment of the object of theinvention.

The hydrocarbon plasticizer [B] desirably has a boiling point higherthan the melting point of the high molecular weight polyethylene [A],and the former is preferably higher by 10° C. than the latter. In thisconnection, the melting point of the hydrocarbon plasticizer [B] ispreferably not more than 110° C.

The hydrocarbon plasticizer [B] as mentioned above preferably includesthose having a molecular weight of not more than 2,000 and being solidat room temperature. Of these hydrocarbon plasticizers, especiallypreferred are paraffin waxes having a molecular weight of from 400 to1,000. The hydrocarbon plasticizer being solid at room temperature andhaving a molecular weight of not more than 2,000 gives a homogeneousmixture when melt kneaded at a temperature above 110° C. with the highmolecular weight polyethylene [A]. In this connection, the paraffinwaxes having a molecular weight of 400-1,000 are excellent indispersibility at the time when melt kneaded with the high molecularweight polyethylene [A].

The paraffin waxes include n-alkanes having not less than 22 carbonssuch as docosane, tricosane, tetracosane and triacontane, or mixtures oflower n-alkanes containing the above-mentioned n-alkanes as mainingredients; the so-called paraffin waxes formed by separation frompetroleum; low molecular weight polymers of ethylene or copolymers ofethylene and α-olefin of 3 or more carbon atoms such as medium lowpressure polyethylene, high pressure polyethylene waxes and ethylenecopolymer waxes; and waxes obtained by reducing molecular weight ofpolyethylenes such as medium low pressure polyethylenes and highpressure polyethylene by heat degradation, and oxidized or modifiedwaxes such as oxidation products or modification products of theabove-mentioned waxes.

The raw fabric sheet is prepared by melt kneading 20-75 parts by weight,preferably20-50 parts by weight and especially 20-40 parts by weight ofthe high molecular weight polyethylene [A] together with 80-25 parts byweight, preferably 80-50 parts by weight and especially 80-60 parts byweight of the hydrocarbon plasticizer [B], and forming the resultingmolten product into a sheet, followed by cooling to solidify.

The high molecular weight polyethylene [A] and the hydrocarbonplasticizer [B] may be melt kneaded by kneading with a kneading device,e.g., a Henschel mixer, V-blender, ribbon blender or tumbler blender,followed by melt kneading by means of a screw extruder such as a singlescrew extruder or twin-screw extruder, or by means of a kneader orBanbury mixer. The melt kneading temperature employed in.-that case isusually higher than the melting point of the high molecular weightpolyethylene [A], and the temperature is not more than 300° C.,preferably 160°-250° C. If the melt kneading temperature employed islower than the melting point of the high molecular weight polyethylene,the resulting kneaded product increases in viscosity and hence there isa case wherein no homogeneous kneading can be performed. When the meltkneading is carried out at a temperature above 300° C., the highmolecular weight polyethylene [A] sometimes undergoes heatdeterioration.

The resulting mixture of the high molecular weight polyethylene [A] andthe hydrocarbon plasticizer [B] may be formed into a raw fabric sheet bymeans of the extrusion technique using an extruder equipped with a T-dieor by means of the compression molding.

When the raw fabric sheet is formed by the extrusion technique, theremay be employed a continuous method wherein the raw sheet is extrudedthrough the T-die while the mixture is melt kneaded by means of a screwextruder.

When the raw sheet is formed by means of the compression molding, themelt kneading of the mixture is carried out separately in advance, andthe molten mixture obtained is then formed into a sheet-like shape bymeans of the compression molding.

The thickness of the raw sheet is desirably 0.05-5 mm, because saidsheet is nipped at the time of the stretching thereof by means of atenter clip.

In the present specification, the melting point is represented by avalue measured according to ASTM D3417 with a differential scanningcalorimeter (DSC) unless otherwise specified.

Extraction Of Hydrocarbon Plasticizer [B]

Subsequently, the hydrocarbon plasticizer [B] is extracted with asolvent [C] capable of dissolving said plasticizer [B] from the rawfabric sheet and removed therefrom at a temperature of 90° C.,preferably at a temperature of 80° C. to obtain an unstretched sheetcontaining substantially no hydrocarbon plasticizer [B].

Useful in the invention as the solvent [C] capable of extracting andremoving the hydrocarbon plasticizer from the raw fabric sheet is a lowmolecular weight hydrocarbon solvent such as n-hexane, cyclohexane,n-heptane, n-octane, n-decane or n-dodecane.

The extraction and removal of the hydrocarbon plasticizer [B] from theraw fabric sheet is carried out desirably at a temperature above amelting point of the hydrocarbon plasticizer [B] if said plasticizer hasa melting point. By carrying out the extraction and removal of thehydrocarbon plasticizer [B] from the raw sheet at a melting point ofsaid plasticizer, the time required therefor can be shortened. In thisconnection, the upper limit of the treatment time of the extraction andremoval of the hydrocarbon plasticizer is the softening point of the rawsheet. The softening point of the raw sheet is usually about 90° C.though it more or less varies depending upon the kind of the hydrocarbonplasticizer used, or upon the blending proportion of the high molecularweight polyethylene [A] and the hydrocarbon plasticizer [B]. If theextraction and removal operation is carried out at a temperature higherthan the softening point of the raw sheet for a long period of time, anexcellent structure suitable for the biaxial stretchability of the rawsheet sometimes undergoes change, said structure being formed by thecrystallization of the high molecular weight polyethylene in thepresence of the hydrocarbon plasticizer [B].

It is desirable to carry out the extraction and removal of thehydrocarbon plasticizer [B] from the raw fabric sheet in a constrainedstate (fixed end). If the extraction and removal of the said plasticizeris carried out in an unconstrained state (free end), the unstretchedsheet (raw sheet) tends to warp or crease, whereby it often troublesometo fit the unstretched sheet with a tenter. When the extraction andremoval of the hydrocarbon plasticizer [B] is carried out in theunconstrained state (free end), the raw sheet undergoes contraction.However, no stretching characteristics of the sheet are marred, if saidcontraction is up to 30% in terms of the specific surface area.

By the, unstretched sheet containing substantially no residualhydrocarbon plasticizer as referred to in the invention is meant thatthe content in said unstretched sheet of the hydrocarbon plasticizer [B]after the extraction and removal thereof is not more than 5% by weightbased on the weight of said unstretched sheet.

The presence of the hydrocarbon plasticizer [B] remaining in theunstretched sheet after the extraction and removal treatment may beconfirmed by means of a differential scanning calorimeter (DSC) if saidhydrocarbon plasticizer [B] is of a crystalline structure. Besides thesame confirmation may be accomplished by using a Soxhlet's extractor,wherein the unstretched sheet is treated with a suitable solventincapable of dissolving the high molecular weight polyethylene butcapable of dissolving the hydrocarbon plasticizer [B], for example,boiling n-hexane, when said hydrocarbon plasticizer is paraffin wax, anda loss in weight of the thus treated unstretched sheet is obtained toconfirm whether the hydrocarbon plasticizer [B] still remains in saidsheet or not.

Stretching

Subsequently, the above-mentioned unstretched sheet is stretched at atemperature below 135° C. so that the stretch ratio in the lengthwisedirection becomes 3 times or more and that in crosswise directionbecomes 3 times or more, thereby obtaining a stretched film (biaxiallyoriented film) having a specific surface area of not less than 70 m² /gand a fibril structure.

The temperature at which the unstretched sheet is stretched is not lessthan 60° C. and less than 135° C., preferably 90°-130° C. If thistemperature is not more than 60° C., it becomes difficult to stretch theunstretched sheet to the stretch ratio of three times or more, becausethe stretching stress of said sheet becomes larger, and there issometimes a case wherein the stretched film obtained does not exhibit ahigh modulus of elasticity and high strength. If the stretchingtemperature employed exceeds 135° C., there is sometimes a case whereinthe stretched film becomes a non-porous, dense and gas-impermeable filmand does not have a nonwoven fabric-like gas permeable structure.

The stretch ratio to be employed in that case is 3 times or more,preferably 4 times or more, further preferably 5-20 times and especially6-20 times in the lengthwise direction, and3 times or more, preferably 4times or more, further preferably 5-20 times and especially 6-20 timesin the crosswise direction.

When an ultra-thin (film thickness of not more than 1 μm) stretched filmis prepared, it is desirable to employ the stretch ratio of 20 times ormore in the lengthwise direction as well as in the crosswise direction.If the stretch ratio exceeds 20 times, the thickness of the resultingstretched film becomes usually 1/400 or less of that of the raw fabricsheet, though the thickness varies depending on the porosity of thestretched film and on the composition of the raw sheet.

The process for stretching the aforementioned unstretched sheet includesa simultaneous or successive biaxial stretching according to thetentering method, or a successive biaxial stretching process comprisinga combination of the stretching by a roll in the lengthwise directionand the stretching by a tenter in the crosswise direction. When thestretch ratio in the lengthwise direction is 6 times or more, thestretching in the lengthwise direction is preferably carried out by amulti-stage stretching technique. When the multi-stage stretchingtechnique is employed, the stretching temperature to be employed ispreferably allowed to rise as the stretching proceeds from the forwardstage to the backward stage within the temperature range not exceeding135° C.

The stretched film thus obtained assumes an opaque or translucent white.The light transmittance of this film, though it depends on the thicknessof the film, is not more than 10%, preferably not more than 5%. Thisstretched film has an unwoven fabric-like structure of microfibrilswhich are the minimal structural units of polyethylene.

The stretched film desirably has a specific surface area of 70 m² /g ormore, preferably 90 m² /g. If the specific surface area is not more than70 m² /g, there are some cases where the stretched film, when heated,loses its gas permeability or fails to exhibit physical properties suchas tensile strength.

The stretched film obtained by way of such stretching processes asmentioned above desirably has a porosity of 35-80%. Furthermore, thetensile strength in all directions 0.1GPa or more, preferably 0.2 GPa ormore, and the tensile modulus is 0.5 GPa or more, preferably 1.0 GPa ormore in all directions. Still further, the elongation at breakage of thestretched film is 15% or more, preferably 30% or more and especially 50%or more.

This stretched film has the coefficient of static friction of usually1.5 or more, and the coefficient of kinetic friction of usually 1.5 ormore.

The interlaminar peel strength as measured by the 180° peeling method isnot more than 30 g/cm when an adhesive tape having a peel strength of300 g/cm as measured by the 180° peeling method, said peel strengthbeing measured by peeling off two adhesive tapes having been adhered toeach other on their adherent surfaces according to the 180° peelingmethod, is adhered to the surface of the biaxially oriented film of highmolecular weight polyethylene having the same width as that of theadhesive tape and peeled off therefrom.

The number of Gurley seconds is 0.1-60 seconds, preferably 0.5-30seconds especially 1-15 seconds.

The piercing strength is usually 30-60 g.

The degree of orientation obtained by X-ray diffraction of the stretchedfilm is 0.75 or more, preferably 0.80 or more in all directions on thefilm surface, and the degree of crystallinity is 55% or more, preferably65% or more.

The thickness of the stretched film is usually 500 μm or less,preferably 100-10 μm, though it may be suitably decided according to thepurpose for which the stretched film is used.

Heat treatment

Subsequently, the thus obtained stretched film is heated under astandard length constraint at a temperature of 132°-145° C. for 1 secondto 10 minutes to reduce the specific surface area of said stretched filmby 20 m² /g or more, whereby preparing a biaxially oriented film of highmolecular weight polyethylene. The thus prepared biaxially oriented filmof high molecular weight polyethylene has such characteristics asmentioned above.

By standard length constraint as used in the invention is meant such astate wherein the film is not substantially contracted nor expanded bothin lengthwise and crosswise directions. However, the contraction up toabout 10% in both lengthwise and crosswise directions is allowed at thetime of heat treatment of the stretched film.

A heat medium for heating the stretched film includes such gases as airand nitrogen gas; and such liquids which do not dissolve or modifypolyethylene films, for example, as water, diethylene glycol andtriethylene glycol.

The stretched film is heated desirably under such conditions that thespecific surface area of said stretched film is reduced by 20 m² /g ormore, preferably 20-30 m² /g, and also that the specific surface area ofthe biaxially oriented film of high molecular weight polyethyleneobtained after heat treatment becomes 30-100 m² /g.

If the reduction in specific surface are of the resulting biaxiallyoriented film of high molecular weight polyethylene is less than 20 m²/g, said film sometimes becomes poor in physical properties such asresistance to stratiform separation and surface smoothness. If thespecific surface area of the resulting biaxially oriented film of highmolecular weight polyethylene is less than 30 m² /g no biaxiallyoriented film of high molecular weight polyethylene is obtained.

The temperature employed for heating the stretched film is usually132°-145° C., preferably 132-142 and especially 135°-140° C., and theheating time is 1 second to 10 minutes, preferably 1 second to 60seconds. When heat treated at a temperature of less than 132° C. thestretched film treated thereat sometimes is not improved in physicalproperties. On the other hand, if the stretched film is heat treated ata temperature exceeding 145° C., there are some cases wherein thetreated stretched film loses its nonwoven fabric-like structure and gaspermeability, and is transparentized.

Such heat treatment of the stretched films mentioned above may becarried out under substantially the standard length constraint at atemperature of 132°-145° C. in succession to the stretching step, or maybe carried out under the standard length constraint at a temperature of132°-145° C. after cooling the stretched film once to a temperature ofnot more than 60° C.

As the result of such heat treatment of the stretched film as mentionedabove, the bulk density of the resulting biaxially oriented film of highmolecular weight polyethylene decreases a little than that of thestretched film prior to the heat treatment, and the film thickness ofsaid biaxially oriented film becomes thinner. However, the resultingbiaxially oriented film of high molecular weight polyethylene isimproved in tensile strength by about 30% or more in comparison withthat of said film prior to the heat treatment.

By heating the stretched film under a standard length constraint andspecific conditions as mentioned above, there can be obtained abiaxially oriented film of high molecular weight polyethylene havingdecreased coefficient of static friction and kinetic friction andimproved resistance to stratiform separation and self-form retentivitywhile maintaining the nonwoven fabric-like structure and gaspermeability owned by the aforesaid stretching step.

In the biaxially oriented film of high molecular weight polyethylene asillustrated above, a contact angle to water is usually 110° or more.According to the purpose for which the film is used, however, said filmmust have an affinity for water, and in such a case, the affinity forwater can be improved by such a method as mentioned below.[Surface-modified biaxially oriented film of high molecular weightpolyethylene and process for the preparation thereof]

The surface-modified biaxially oriented film of high molecular weightpolyethylene of the invention and a process for the preparation thereofare illustrated hereinafter.

The surface-modified biaxially oriented film of high molecular weightpolyethylene according to the invention comprising a high molecularweight polyethylene having an intrinsic viscosity [η] of at least notless than 5.0 dl/g, in which

(i) an average pore diameter of 0.1-10 μm,

(ii) a void content of 20-70%,

(iii) a tensile modulus is not less than 0.6 GPa,

(iv) a tensile strength is not less than 0.1 GPa,

(v) a coefficient of static friction is not more than 1.0,

(vi) a coefficient of kinetic friction is not more than 1.0, and

(vii) a contact angle of the film surface to the water drop is not morethan 90°,

and has a nonwoven fabric-like and gas permeable structure ofmicrofibrils.

The surface-modified biaxially oriented film of high molecular weightpolyethylene of the invention has the same characteristics as in theaforesaid biaxially oriented film of high molecular weight polyethyleneof the invention and is excellent also in affinity for water. Theaffinity for water may be represented by means of a contact angleutilizing a surface tension of water, and the contact angle to the waterof this surface-modified biaxially oriented film of high molecularweight polyethylene is not more than 90°, preferably not more than 80°and especially not more than 70°.

The measurement of contact angle to water may be carried out, forexample, by means of an automatic contact angle meter of CA-Z Modelmanufactured and sold by Kyowa Kaimen Kagaku K.K.

The surface-modified biaxially oriented film of high molecular weightpolyethylene as illustrated above may be prepared by subjecting theaforesaid biaxially oriented film of high molecular weight polyethyleneof the invention to surface treatment such as corona dischargetreatment, plasma discharge treatment or electron radiation treatment,or polymerizing a vinyl monomer having a hydrophilic group on thesurface of said biaxially oriented film.

The methods for polymerizing a vinyl monomer having a hydrophilic groupon the surface of the biaxially oriented film to obtain thesurface-modified biaxially oriented film of high molecular weightpolyethylene include a method comprising applying the vinyl monomerhaving the hydrophilic group on the surface of the biaxially orientedfilm and then carrying out an electron ray irradiation thereon.

Concrete examples of the vinyl monomer having a hydrophilic groupinclude unsaturated carboxylic acids such as acrylic acid andmethacrylic acid; carboxylic acid vinyl esters such as vinyl acetate;and mixtures thereof.

EFFECT OF THE INVENTION

The biaxially oriented films of high molecular weight polyethylene ofthe invention are excellent in tensile modulus of elasticity and tensilestrength, low in coefficients of static and kinetic friction, andexcellent in resistance to interlaminar separation and piercing strengthand also in gas permeability.

The surface modified biaxially oriented films of high molecular weightpolyethylene of the invention may be obtained by imparting affinity forwater to the above-mentioned biaxially oriented films of high molecularweight of polyethylene.

The biaxially oriented films of high molecular weight polyethylene andsurface-modified biaxially oriented films of high molecular weightpolyethylene of the invention as illustrated above may be used eachsingly or as a laminated film composed of a plurality of homogeneousfilms, or may be used in lamination with films of various kinds ofresins including polyethylene, paper, cellophane or aluminum foil.

Concretely, these films of two types according to the invention may beused, for example, as a support of useful filter for separatingultrafine particle materials from various liquids containing the same,or separating filters. Further, they can be used as condenser film,insulating paper or battery separator. Furthermore, they can be used,making the best use of their high strength, light weight properties,water resistance and good printability, as outdoor displaying printpaper, various kinds of packaging materials, especially air mail envelopand packaging materials. Furthermore, utilizing their gas permeability,they can be used as packaging materials for moisture or oxygenabsorbers.

The present invention is illustrated below in more detail with referenceto examples, but it should be construed that the invention is in no waylimited to those example.

In the following examples, "parts" and "%" are individually "parts byweight" and "% by weight" unless otherwise stated.

EXAMPLE 1

A particulate mixture obtained by mixing uniformly together 30 parts byweight of a particulate high molecular weight polyethylene (intrinsicviscosity [η]=16 dl/g), 70 parts by weight of particulate paraffin wax(m.p.=69° C., molecular weight 460) and 0.5%, based on the highmolecular weight polyethylene, of 3,4-di-tert-butyl-4-hydroxytoluene asa process stabilizer was melt kneaded by means of a twin screw-type meltkneading machine Laboplastomill (20R-200 Model of Toyo Seiki Seisakusho)at 190° C. for 10 minutes to obtain a uniform molten mixture. In thatcase, the number of revolutions of the screws was 50 r.p.m.

This molten mixture was separated in the molten state, placed, as itwas, between a pair of press plates, adjusted in thickness by means of ametal frame, and immediately thereafter put between hot plates of a hotpress preset at 190° C. whereby the molten mixture was processed into asheet form. After 5-minute compression by means of the hot press, themolten mixture held between the press plates was compressed by means ofa cold press adjusted to 20° C., cooled and solidified to obtain a rawfabric sheet of about 1 mm in thickness.

Subsequently, in order to prevent contraction, the raw fabric sheet wasfixed by means of a pair of metal frames wherein said raw fabric sheetwas held, and allowed to stand for about 2 hours in a bath of n-decanekept at 60° C., whereby the paraffin wax was extracted and removed fromsaid raw sheet. In that case, the n-decane was stirred in order tofacilitate the extraction and removal of the paraffin wax.

After that, the unstretched sheet held between the metal frames wasdried under reduced pressure at room temperature (23° C.). The thusdried unstretched sheet observed by means of a differential scanningcalorimeter (DSC), whereupon no melting point of the paraffin wax at 69°C. was recognized.

Subsequently, the unstretched sheet thus dried was subjected tosimultaneous biaxial stretching (length×breadth: 6×6) at a stretchingtemperature of 120° C. to obtain a stretched film (sample No. 1) of 14.8μm in thickness. In that case, there was used a tentering type biaxialstretching machine of Heavy Model of Toyo Seiki Seisakusho K.K.

The thus obtained stretched film (sample No.1) was fixed by means of ametal frames so that said film will not contract, and heated, under sucha standard length constraint, for 10 minutes by means of an air ovenpreset to a temperature of 140° C. to obtain a biaxially oriented filmof high molecular weight polyethylene (sample No. 2).

Results obtained are shown in Table 1.

EXAMPLE 2

A stretched film of 11.5 μm in thickness (sample No. 3) was obtained byrepeating Example 1 except that a high molecular weight polyethylenehaving an intrinsic viscosity [η] of 8.4 l/g was used in place of thehigh molecular weight polyethylene used in said Example 1.

Subsequently, the thus obtained stretched film (sample No. 3) washeated, under a standard length constraint, for 10 minutes by means ofan air oven preset to a temperature of 140° C. to obtain a biaxiallyoriented film of high molecular weight polyethylene (sample No. 4).

Results obtained are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                      Example 1 Example 2                                             ______________________________________                                        Sample No.      1      2        3    4                                        Sample No. of test                                                                            1      1        3    3                                        material to be heat                                                           treated                                                                       Presence of heat                                                                              No     Yes      No   Yes                                      treatment                                                                     Tensile strength                                                                              0.34   0.62     0.08 0.13                                     (GPa)                                                                         Tensile modulus of                                                                            1.66   2.95     1.20 2.37                                     elasticity (GPa)                                                              Elongation at   73     73       8    26                                       failure (%)                                                                   Porosity (%)    59     39       40   37                                       Number of Gurley                                                                              3.8    8.4      5.3  16.2                                     seconds (Second)                                                              Piercing strength                                                                             52.0   49.2     30.7 36.3                                     (g)                                                                           Specific surface                                                                              78     51       82   57                                       area (m.sup.2 /g)                                                             Coefficient of  1.75   0.41     1.65 0.47                                     kinetic friction                                                              Coefficient of  2.29   0.43     2.37 0.51                                     static friction                                                               Interlaminar    27.3   65.3     26.2 60.3                                     peeling strength                                                              (g/cm)                                                                        Average pore    0.6    0.3      0.7  0.4                                      diameter (μm)                                                              ______________________________________                                    

EXAMPLE 3

The biaxially oriented film of high molecular weight polyethylene(sample No. 2) obtained in Example 1 was subjected to surface treatmentin the manner as described below.

The biaxially oriented film of high molecular weight polyethylene(sample No. 2) was immersed in glacial acrylic acid (special gradereagent of Wako Junyaku K.K.), whereupon the immersed film (sample No.2) became translucent.

Successively, the resulting translucent and biaxially stretched film ofhigh molecular weight polyethylene (sample No. 2) was air dried until itbecame white, and the thus dried film was then irradiated with electronrays to obtain a surface-modified biaxially oriented film of highmolecular weight polyethylene (sample No. 5).

The irradiation of said dried film with the electron rays was carriedout using an electron ray irradiating apparatus (Curetoron Type TBC200-20-10 of Nisshin High Voltage K.K.) at a room temperature (23° C.)in a nitrogen atmosphere. The irradiation conditions employed in thatcase included an accelerating voltage of 200 KV and an irradiation doseof 3 Mrad.

The amount of the glacial acrylic acid coated was obtained from theweight of the film measured after the irradiation with electron rays.

Results obtained are shown in Table 2.

EXAMPLE 4

A surface-modified biaxially oriented film of high molecular weightpolyethylene (sample No. 6) was obtained by repeating Example 3 exceptthat the biaxially oriented film of high molecular weight polyethylene(sample No. 4) obtained in Example 2 was used in place of the film(sample No. 2) used in said Sample 3.

Results obtained are shown in Table 2.

EXAMPLE 5

The biaxially oriented film of high molecular weight polyethylene(sample No. 2) obtained in Example 1 was subjected to surface treatmentwith plasma discharge in the following procedure under the conditionsshown in Table 2 to obtain surface-treated and biaxially oriented filmsof high molecular weight polyethylene (sample Nos. 7 and 8).

In carrying out the plasma discharge surface treatment of the film(sample No. 2), a plasma discharge apparatus (a product of Toshiba whichcomprising a microwave generator Model TMG 132F, a plasma generatingapplicator Model TAW 173 B500, a Three-Stub Tube Model TMU 298) wasused, wherein the pressure employed was 0.7 Torr, and the atmosphere gaswas oxygen.

Results obtained are shown in Table 2.

EXAMPLE 6

Surface-modified biaxially oriented films of high molecular weightpolyethylene (sample Nos. 9 and 10) were obtained by the plasmadischarge surface treatment in the same manner as in Example 5 exceptthat the biaxially oriented film of high molecular weight polyethylene(sample No.4) obtained in Example 2 was used and the surface treatmentconditions employed were as shown in Table 2.

Results obtained are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                Example                                                                              Example   Example  Example                                             3      5         4        6                                           ______________________________________                                        Sample No.                                                                              5        7      8    6      9    10                                 Sample No. of                                                                           2        2      2    4      4    4                                  test material to                                                              be heat treated                                                               Surface treat-                                                                          #1       #2     #3   #4     #5   #6                                 ment conditions                                                               Contact angle                                                                           87       64     67   83     78   64                                 to water (°)                                                           Tensile strength                                                                        0.61     0.58   0.60 0.13   0.11 0.10                               (GPa)                                                                         Tensile modulus                                                                         2.91     2.67   2.83 2.31   2.20 2.08                               of elasticity                                                                 (GPa)                                                                         Elongation at                                                                           73       68     70   24     21   20                                 failure (%)                                                                   Porosity (%)                                                                            33       38     38   36     36   35                                 Number of 7.8      8.3    8.5  17.3   17.2 15.3                               Gurley seconds                                                                (Second)                                                                      Piercing  48.0     43.0   46.0 35.0   31.0 30.0                               strength (g)                                                                  Specific surface                                                                        43       53     51   50     56   58                                 area (m.sup.2 /g)                                                             Coefficient of                                                                          0.39     0.43   0.43 0.40   0.48 0.49                               kinetic friction                                                              Coefficient of                                                                          0.42     0.45   0.47 0.45   0.52 0.55                               static friction                                                               Interlaminar                                                                            62.5     59.3   60.2 59.1   58.8 55.3                               peeling strength                                                              (g/cm)                                                                        Average pore                                                                            0.2      0.3    0.3  0.3    0.4  0.4                                diameter (μm)                                                              ______________________________________                                         #1: Amount of acrylic acid coated 5%, Electron ray irradiation treatment      #2: Plasma discharge treatment 1 KW, 10 seconds                               #3: Plasma discharge treatment 0.5 KW, 10 seconds                             #4: Amount of acrylic acid coated 3%, Electron ray irradiation treatment      #5: Plasma discharge treatment 1 KW, 30 seconds                               #6: Plasma discharge treatment 1 KW, 1 minute                            

Comparative Example 1

Unstretched films (sample Nos. 11 and 12) were obtained by repeatingExample 1 except that a high molecular weight polyethylene having anintrinsic viscosity [η] of 8.4 dl/g was used in such proportion as shownin Table 3.

After removing the paraffin wax by extraction from the unstretched films(sample Nos. 11 and 12) containing the same, respectively, the thustreated films were individually subjected to simultaneous biaxialstretching at a stretch ratio as shown in Table 3 and a stretchingtemperature of 120° C. to obtain stretched films (sample Nos. 13-16).The biaxial stretching machine used was the same as used in Example 1.

Results obtained are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Comparative Example 1                                                         ______________________________________                                        Sample No.     11    13     14   12   15   16                                 Amount of Polyethylene                                                                       20    20     20   30   30   30                                 contained (%)                                                                 Stretch ratio --     4 ×                                                                            6 ×                                                                          --   4 ×                                                                          6 ×                          Length × Width (time)                                                                        4      6         4    6                                  Film thickness (μm)                                                                      730    12.7   8.0  830  24.6 11.5                               Tensile strength (GPa)                                                                      --     0.07   0.09 --   0.06 0.08                               Tensile modulus of                                                                          --     0.47   0.81 --   0.91 1.20                               elasticity (GPa)                                                              Elongation at failure                                                                       --     15     12   --   8    8                                  (%)                                                                           Porosity (%)  --     28     49   --   37   40                                 Number of Gurley                                                                            --     2.3    3.2  --   4.7  5.3                                seconds (second)                                                              Piercing strength (g)                                                                       --     25.3   26.7 --   27.1 30.7                               Specific surface area                                                                       --     52     90   --   54   82                                 (m.sup.2 /g)                                                                  Coefficient of kinetic                                                                      --     1.32   1.57 --   1.41 1.65                               friction                                                                      Coefficient of static                                                                       --     2.44   2.45 --   2.22 2.37                               friction                                                                      Interlaminar peeling                                                                        --     25.1   24.3 --   29.1 26.2                               strength (g/cm)                                                               Average pore diameter                                                                       --     3      3.1  --   1    0.7                                (μm)                                                                       Number of stretching                                                                        --     1      1    --   1    1                                  steps                step   step      step step                               ______________________________________                                    

Comparative Example 2

A high molecular weight polyethylene (intrinsic viscosity [η]=16 dl/g)and a paraffin wax (m.p.=69° C., molecular weight 460) were treated inthe same manner as in Example 1 to obtain a raw fabric sheet of about 1mm in thickness. This raw sheet contained 30% of the high molecularweight polyethylene.

Subsequently, in order to prevent its contraction, the thus obtained rawfabric sheet was fixed by putting it between a pair of metal frames, andthe fixed sheet was allowed to stand for about 5 minutes in an n-decanebath kept at 60° C., whereby the paraffin wax was removed by extractionfrom the raw sheet. In that case, the n-decane was stirred in order tofacilitate the removal by extraction of the paraffin wax.

Thereafter, the unstretched sheet fixed by means of metal framed wasdried under reduced pressure at room temperature (23° C.). Theunstretched film thus dried (sample No. 17) was observed by means of adifferential scanning calorimeter (DSC), whereupon a melting point ofthe paraffin wax was noticed at 69° C. The dried unstretched sheet wasthen cut into pieces of about 2-3 mm, and these pieces were treated witha Soxhlet's extracter using n-hexane as an extraction solvent, wherebythe residual paraffin wax was completely removed from the unstretchedsheet The residual amount of the paraffin wax obtained from a differencein weight between the samples prior and after the Soxhlet extraction was8% based on the weight of the dried unstretched sheet.

Subsequently, this unstretched sheet was subjected to the same biaxialstretching as in Example 1.

The biaxially oriented film thus obtained was fixed by means of metalframes in two directions, washed with n-hexane to remove the residualparaffin and dried at room temperature (23° C.) to obtain biaxiallyoriented films (samples Nos. 18 and 19). Results are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        Comparative Example 2                                                         ______________________________________                                        Sample No.       17       18      19                                          Amount of Polyethylene                                                                         30       30      30                                          contained (%)                                                                 Stretch ratio   --        4 × 4                                                                           6 × 6                                 Length × Width (time)                                                   Film thickness (μm)                                                                        960       25.4    15.4                                        Tensile strength (GPa)                                                                        --        0.09    0.13                                        Tensile modulus of                                                                            --        0.75    1.19                                        elasticity (GPa)                                                              Elongation at failure                                                                         --        63      32                                          (%)                                                                           Porosity (%)    --        23.5    43.1                                        Number of Gurley                                                                              --        35.6    32.2                                        seconds (second)                                                              Piercing strength (g)                                                                         --        27.3    28.0                                        Specific surface area                                                                         --        23      29                                          (m.sup.2 /g)                                                                  Coefficient of kinetic                                                                        --        0.84    0.93                                        friction                                                                      Coefficient of static                                                                         --        1.27    1.52                                        friction                                                                      Interlaminar peeling                                                                          --        23.3    25.4                                        strength (g/cm)                                                               Average pore diameter                                                                         --        0.1>    0.1>                                        (μm)                                                                       Number of stretching                                                                          --        1 step  1 step                                      steps                                                                         ______________________________________                                    

Comparative Example 3

The stretched film (sample No. 1) obtained in Example 1 was fixed bymeans of metal frames so that the film will not contract, and the fixedfilm was heated under such a standard length constraint for 1 minute bymeans of air oven preset at a temperature of 155° C. After heating thefilm became transparent partially, and the transparent portion of thefilm was observed under a scanning type electron microscope, whereuponno microporous structure of the film was noticed.

Comparative Example 4

The stretched film (sample No. 14) obtained in Comparative Example 1 wasfixed by means of metal frames so that the film will not contract, andthe fixed film was heated under such a standard length constraint for 1minute by means of an air oven preset at a temperature of 155° C. Afterheating, the film became transparent partially, and the transparentportion of the film was observed under a scanning type electronmicroscope, whereupon no microporous structure of the film was noticed.

EXAMPLE 7

A particulate mixture obtained by mixing uniformly together 30% byweight of a particulate high molecular weight polyethylene (intrinsicviscosity [η]=16 dl/g), 70% by weight of particulate paraffin wax(m.p.=69° C., molecular weight 460) and 0.5%, based on the highmolecular weight polyethylene, of 3,4-di-tert-butyl-4-hydroxytoluene asa process stabilizer was melt kneaded by means of a twin screw-type meltkneading machine Laboplastomill (20R-200 Model of Toyo Seiki Seisakusho)at 190° C. for 10 minutes to obtain a uniform molten mixture. In thatcase, the number of revolutions of the screws was 50 r.p.m.

This molten mixture was separated in the molten state, placed, as itwas, between a pair of press plates, adjusted in thickness by means of ametal frame, and immediately thereafter put between hot plates of a hotpress preset at 190° C., whereby the molten mixture was processed into asheet form. After 5-minute compression by means of the hot press, themolten mixture held between the press plates was compressed by means ofa cold press adjusted to 20° C., cooled and solidified to obtain a rawfabric sheet of about 780 μm in thickness.

Subsequently, in order to prevent contraction, the raw fabric sheet wasfixed by means of a pair of metal frames wherein said raw fabric sheetwas held, and allowed to stand for about 2 hours in a bath of n-decanekept at 60° C., whereby the paraffin wax was extracted and removed fromsaid raw sheet. In that case, the n-decane was stirred in order tofacilitate the extraction and removal of the paraffin wax.

After that, the unstretched sheet held between the metal frames wasdried under reduced pressure at room temperature (23° C.).

Subsequently, the unstretched sheet thus dried was subjected to biaxialstretching to obtain a stretched film (sample Nos. 20-21).

In that case, there was used a tentering type biaxial stretching machineof Heavy Model of Toyo Seiki Seisakusho K.K.

When the sheet is subjected to simultaneous stretching, the lengthwisestretching was carried out simultaneously with the crosswise stretchinguntil desired stretch ratio was attained. When the film is subjected tosuccessive stretching, the lengthwise stretching was carried out whilemaintaining the film in a definite width in the cross wise direction,and the crosswise stretching was then carried out while maintaining adefinite width of the film stretched in the lengthwise direction.

The conditions under which the samples were biaxially stretched areshown in Table 5.

                  TABLE 5                                                         ______________________________________                                                            Stretching  Stretching                                             Stretching temperature ration (time)                                 Sample No.                                                                             method     (°C.)                                                                              Length                                                                              Width                                   ______________________________________                                        20       Simultaneous                                                                             120         6     6                                                biaxial                                                                       stretch                                                              21       Successive 120         6     6                                                biaxial                                                                       stretch                                                              ______________________________________                                    

The stretched films (sample Nos. 20-21) obtained under the conditions inTable 5 were fixed by means of a pair of metal frames and subjected toheat treatment under the conditions as shown in Table 7.

The heat treatment was performed by using an oven (Perfect Oven:manufactured and sold by Tobai Seisakusho).

Results obtained are shown in Tables 6-7.

                                      TABLE 6                                     __________________________________________________________________________    Example 7 (sample No.20)                                                      __________________________________________________________________________    Heating   -- 150 140 140 140 130 130 130 120                                  temperature (°C.)                                                      Heating time                                                                            -- 60  600 60  10  600 60  10  600                                  (Sec)                                                                         Film thickness                                                                          25.0                                                                             15.7                                                                              16.2                                                                              17.1                                                                              18.5                                                                              20.3                                                                              20.5                                                                              22.3                                                                              24.9                                 μm                                                                         Tensile   0.22                                                                             0.21                                                                              0.40                                                                              0.38                                                                              0.34                                                                              0.35                                                                              0.33                                                                              0.29                                                                              0.28                                 strength                                                                      (GPa)                                                                         Tensile   1.2                                                                              1.1 1.6 1.5 1.4 1.3 1.3 1.2 1.1                                  modulus of                                                                    elasticity (GPa)                                                              Elongation at                                                                           73 53  85  82  80  81  80  75  73                                   failure (%)                                                                   Porosity (%)                                                                            55.5                                                                             32.2                                                                              39.8                                                                              40.4                                                                              43.2                                                                              44.9                                                                              45.2                                                                              50.1                                                                              55.3                                 Number of Gurley                                                                        7.3                                                                              *   9.8 9.4 8.3 7.9 7.8 7.6 7.2                                  seconds                                                                       (Sec.)                                                                        Piercing  45 46  52  47  45  45  43  44  46                                   strength (g)                                                                  Specific  94 30  51  57  63  79  82  91  94                                   surface area                                                                  (m.sup.2 /g)                                                                  Coefficient                                                                             1.69                                                                             0.39                                                                              0.42                                                                              0.45                                                                              0.62                                                                              1.05                                                                              1.13                                                                              1.42                                                                              1.68                                 of kinetic                                                                    friction                                                                      Coefficient                                                                             2.30                                                                             0.40                                                                              0.45                                                                              0.56                                                                              0.84                                                                              1.41                                                                              1.52                                                                              1.93                                                                              2.28                                 of static                                                                     friction                                                                      Interlaminar                                                                            29.0                                                                             70.3                                                                              66.6                                                                              65.3                                                                              60.1                                                                              37.2                                                                              35.3                                                                              35.4                                                                              31.2                                 peeling                                                                       strength (g/cm)                                                               Average pore                                                                            0.6                                                                              <0.1                                                                              0.3 0.3 0.3 0.4 0.4 0.4 0.5                                  diameter (μm)                                                              __________________________________________________________________________     *: 180 seconds or more                                                   

                                      TABLE 7                                     __________________________________________________________________________    Example 7 (sample No.21)                                                      __________________________________________________________________________    Heating  -- 150 140 140 140 130 130 130 120                                   temperature (°C.)                                                      Heating time                                                                           -- 60  600 60  10  600 60  10  600                                   (Sec)                                                                         Film thickness                                                                         43.2                                                                             20.3                                                                              30.1                                                                              34.5                                                                              39.2                                                                              38.6                                                                              40.9                                                                              42.9                                                                              43.0                                  μm                                                                         Tensile strength                                                              (GPa)                                                                         Width    0.22                                                                             0.18                                                                              0.42                                                                              0.38                                                                              0.33                                                                              0.31                                                                              0.31                                                                              0.26                                                                              0.23                                  Length   0.25                                                                             0.20                                                                              0.43                                                                              0.40                                                                              0.37                                                                              0.35                                                                              0.34                                                                              0.28                                                                              0.25                                  Tensile modulus                                                               of elasticity                                                                 (GPa)                                                                         Width    1.1                                                                              0.9 1.5 1.4 1.3 1.2 1.1 1.1 1.1                                   Length   0.8                                                                              0.7 1.6 1.5 1.2 0.9 0.9 0.9 0.8                                   Elongation at                                                                 failure (%)                                                                   Width    74 51  83  80  76  76  74  73  72                                    Length   73 48  81  79  75  73  74  74  73                                    Porosity (%)                                                                           75.1                                                                             43.8                                                                              61.8                                                                              67.0                                                                              71.2                                                                              70.3                                                                              72.5                                                                              68.7                                                                              75.0                                  Number of                                                                              2.7                                                                              *   2.0 2.1 2.2 2.1 2.2 2.3 2.6                                   Gurley seconds                                                                (Sec.)                                                                        Piercing 44 46  49  47  47  46  45  44  44                                    strength (g)                                                                  Specific 121                                                                              42  83  89  98  94  105 117 120                                   surface area                                                                  (m.sup.2 · g)                                                        Coefficient                                                                            1.84                                                                             0.40                                                                              0.43                                                                              0.51                                                                              0.68                                                                              0.94                                                                              1.37                                                                              1.49                                                                              1.81                                  of kinetic                                                                    friction                                                                      Coefficient                                                                            2.73                                                                             0.43                                                                              0.57                                                                              0.62                                                                              0.98                                                                              1.32                                                                              1.87                                                                              2.21                                                                              2.70                                  of static                                                                     friction                                                                      Interlaminar                                                                  peeling strength                                                              (g/cm)                                                                        Width    27.0                                                                             75.8                                                                              64.1                                                                              63.7                                                                              61.5                                                                              37.2                                                                              35.5                                                                              33.4                                                                              27.2                                  Length   25.3                                                                             70.3                                                                              63.2                                                                              59.2                                                                              60.3                                                                              37.3                                                                              34.0                                                                              32.2                                                                              26.4                                  Average pore                                                                           0.9                                                                              <0.1                                                                              0.3 0.3 0.3 0.5 0.5 0.6 0.8                                   diameter (μm)                                                              __________________________________________________________________________     *: 180 seconds or more                                                   

What is claimed is:
 1. A process for preparing a surface-modifiedbiaxially oriented film of high molecular weight polyethylene, whichcomprises:(1) melt kneading together 20-75 parts by weight of a highmolecular weight polyethylene (A) having an intrinsic viscosity (η) ofat least 5.0 dl/g or more and 80-25 parts by weight of a hydrocarbonplasticizer (B) which is a solid at ordinary temperature and capable ofmaking a uniform mixture with the high molecular weight polyethylene(A), followed by forming a raw fabric sheet from the resulting moltenmixture, (2) removing by extraction the hydrocarbon plasticizer (B) fromthe raw fabric sheet with a solvent (C) capable of dissolving saidhydrocarbon plasticizer (B) to obtain an unstretched sheet containingsubstantially no residual hydrocarbon plasticizer (B), (3) stretching ata temperature of less than 135° C. the unstretched sheet so that thestretch ratio in the lengthwise direction becomes 3 times or more andthat in the crosswise direction becomes 3 times or more, therebyobtaining a stretched film having a specific surface area of 70 m² /g ormore and a fibril structure, (4) heating the stretched film under thestandard length constraint at a temperature of 132°-145° C. for 1 secondto 10 minutes, thereby reducing said stretched film in specific surfacearea by 20 m² /g or more to obtain a biaxially oriented film of highmolecular weight polyethylene, and (5) modifying the surface of thebiaxially oriented film of high molecular weight polyethylene bysubjecting the film surface to a surface treatment selected from thegroup consisting of corona discharge treatment, plasma dischargetreatment, electron radiation treatment, and chemical modification, thechemical modification comprising providing a vinyl monomer having ahydrophilic group on the film surface and irradiating with electron raysthe film surface to polymerize the applied vinyl monomer, whereby theresulting film has a contact angle of the film surface to a water dropat a level of 90° or less.
 2. A process for preparing a surface-modifiedbiaxially oriented film of high molecular weight polyethylene, whichcomprises(1) melt kneading together 20-75 parts by weight of a highmolecular weight polyethylene (A) having an intrinsic viscosity (η) ofat least 5.0 dl/g or more and 80-25 parts by weight of a hydrocarbonplasticizer (B) which is a solid at ordinary temperature and capable ofmaking a uniform mixture with the high molecular weight polyethylene(A), followed by forming a raw fabric sheet from the resulting moltenmixture, (2) removing by extraction the hydrocarbon plasticizer (B) fromthe raw fabric sheet with a solvent (C) capable of dissolving saidhydrocarbon plasticizer (B) to obtain an unstretched sheet containingsubstantially no residual hydrocarbon plasticizer (B), (3) stretching ata temperature of less than 135° C. the unstretched sheet so that thestretch ratio in the lengthwise direction becomes 3 times or more andthat in the crosswise direction becomes 3 times or more, therebyobtaining a stretched film having a specific surface area of 70 m² /g ormore and a fibril structure, (4) heating the stretched film under thestandard length constraint at a temperature of 132°-145° C. for 1 secondto 10 minutes, thereby reducing said stretched film in specific surfacearea by 20 m² /g or more to obtain a biaxially oriented film of highmolecular weight polyethylene, and (5) providing a vinyl monomer havinga hydrophilic group on the surface of the biaxially oriented film ofhigh molecular weight polyethylene, and irradiating with electron raysthe film surface to polymerize the applied vinyl monomer, thereby makingthe resulting film have a contact angle of the film surface to a waterdrop at a level of 90° or less.
 3. The process of claim 2 wherein thehigh molecular weight polyethylene (A) has an intrinsic viscosity (η) offrom about 10 dl/g to about 30 dl/g.
 4. The process of claim 2 whereinthe hydrocarbon plasticizer (B) has a boiling point higher than themelting point of the high molecular weight polyethylene (A) by at leastabout 10° C. and a melting point of not more than about 110° C.
 5. Theprocess of claim 2 wherein the hydrocarbon plasticizer (B) is a paraffinwax having a molecular weight of from about 400 to 1,000.
 6. The processof claim 2 wherein in step (3) stretching is at a temperature of from90° to 130° C. and in step (4) heating under the standard lengthconstraint is at a temperature of 135° to 140° C.
 7. The process ofclaim 2 which further comprises, between step (3) of stretching at atemperature of less than 135° C., and step (4) of heating the stretchedfilm under the standard length constraint, a step of cooling thestretched film to a temperature of not more than 60° C.
 8. A process forpreparing a surface-modified biaxially oriented film of high molecularweight polyethylene, which comprises(1) melt kneading together 20-75parts by weight of a high molecular weight polyethylene (A) having anintrinsic viscosity (η) of at least 5.0 dl/g or more and 80-25 parts byweight of a hydrocarbon plasticizer (B) which is a solid at ordinarytemperature and capable of making a uniform mixture with the highmolecular weight polyethylene (A), followed by forming a raw fabricsheet from the resulting molten mixture, (2) removing by extraction thehydrocarbon plasticizer (B) from the raw fabric sheet with a solvent (C)capable of dissolving said hydrocarbon plasticizer (B) to obtain anunstretched sheet containing substantially no residual hydrocarbonplasticizer (B), (3) stretching at a temperature of less than 135° C.the unstretched sheet so that the stretch ratio in the lengthwisedirection becomes 3 times or more and that in the crosswise directionbecomes 3 times or more, thereby obtaining a stretched film having aspecific surface area of 70 m² /g or more and a fibril structure, (4)heating the stretched film under the standard length constraint at atemperature of 132°-145° C. for 1 second to 10 minutes, thereby reducingsaid stretched film in specific surface area by 20 m² /g or more toobtain a biaxially oriented film of high molecular weight polyethylene,and (5) subjecting the biaxially oriented film of high molecular weightpolyethylene to a surface treatment selected from the group consistingof corona discharge treatment, plasma discharge treatment and electronradiation treatment, thereby making the resulting film have a contactangle of the film surface to a water drop of a level of 90° or less.